Alumina reduction cell and improved anode system therein

ABSTRACT

Improvements in the construction and operation of alumina reduction cells, particularly as regards procedures and equipment for feeding alumina into the bath of such a cell and for collecting and removing anode reaction gases.

Kibby 1 ,lan.30,1973

[54] ALUMINA REDUCTION CELL AND IMPROVED ANODE SYSTEM THEREIN 75lnventor: R6665 M. KibB'ifibitibj'Aia.

[73] Assignee: Reynolds Metals Company,

Richmond, Va.

[22] Filed: Sept. 2, 1970 [21] Appl. No.: 68,939

[52] U.S. Cl. .l ..204/67, 204/245, 204/247, 204/284 [51] Int. Cl...C22d 3/12, C22d 3/02 [58] Field of Search ..204/67, 243-7;

[561 References Cited 7 V UNITED STATES PATENTS 2,917,441 12/1959 Donald..204/247 X 3,322,658 5/1967 Sem 1.

3,192,140 6/1965 Zorzenoni ..204/247 X 3,243,364 3/1966 Kittlaus et al...204/247 3,551,308 12/1970 Capitaine et al. ..204/067 3,582,483 6/1971Sem ..204/243 R FOREIGN PATENTS OR APPLICATIONS 689,398 6/1964 Canada..204/247 124,627 8/1969 U.S.S.R.... ..204/67 183,953 6/1966U.S.S.R........ ..204/245 45,694 10/1928 Norway ..204/243 R PrimaryExaminer-John H. Mack Assistant ExaminerD. A. Valentine AttorneyGlenn,Palmer, Lyne, Gibbs & Thompson [57] ABSTRACT Improvements in theconstruction and operation of alumina reduction cells, particularly asregards procedures and equipment for feeding alumina into the bath ofsuch a cell and for collecting and removing anode reaction gases.

2% Claims? DiaiiiTg'Figiii-"s PATENTEDJMI 30 I973 sum 10F 3' (BERTINVENTOR R KI ATTO EYS I PATENTEDJAN 30 I975 sum 2 or 3 mvsmon ROBERT M.KIBBY ATTORNEYS PATENTEDJAHBO I975 SHEEF 3 OF 3 H65 Mafia? ATTORNEYSALUMINA REDUCTION CELL AND IMPROVED ANODE SYSTEM THEREIN BACKGROUND OFTHE INVENTION Alumina reduction cells have traditionally employed anodesystems of two different types: pre-baked carbon blocks arrayed in thepot for individual height adjustment and replacement, and Soderberg orself-baking anodes in which a large mass of carbonaceous material,typically a mixture of pitch and coke, is supported in a casing over thecell. In the latter system, the heat of the operating cell causesprogressive baking of the anode.

Then, as the bottom of the anode is consumed during electrolysis, theanode is lowered and more pitch and coke mixture is added to the top ofthe anode to provide a replacement for the bottom portion consumed.

Soderberg type cells are further divided into classes which have eithertop entry electrical contact pins or side pin connections.

With respect to collection of anode gases evolved during operation ofalumina reduction cells, vertical pin Soderberg pots have employed fixedsteel casings around the anode upon which have been mounted skirtsextending over the bath to collect a concentrated form of the gasesevolved from the reduction cell in operation. The major portion of thepitch content of the Soderberg anode migrates downwardly through theanode during operation and is evolved and collected in the skirts. It isthen possible to withdraw the gases from the skirts, burn the pitchfumes to a large extent, and remove a concentrated gas from the pot areato scrubbers where the fluoride effluents can be efficiently removed. Onthe other hand, side pin Soderberg pots have not conventionally usedsuch means to collect concentrated gases. Instead enclosures are madearound the entire anode. These enclosures must be opened during feedingoperations and anode servicing operations, and it has generally beennecessary to remove dilute gases from the pot area resulting in moreexpensive equipment to treat the gases efficiently.

At the same level of power consumption (kilowatt hours per pound),'potsprovided with replaceable prebaked block anode systems typically carrymore than 1.5 times the amperage per square foot of pot shell bottomarea as do pots provided with conventional selfbaking anode systems andmake more than 1.5 times the amount of aluminum.

With so clear-cut an advantage favoring pre-bakes, one might questionwhy self-baking anode systems are used at all. In large measure, theanswer lies in the capital investment which must be made for carbonpresses and anode baking furnaces if a pre-baked anode system is to beemployed, but which isavoided if a self-baking anode system is used.Once conventional self-baking anode system pot lines are built andoperating, their capacity could be increased by investing in press andfurnace equipment and converting to a replaceable pre-baked block anodesystem. However, it would be unusual for the gain in output soobtainable to exceed that which could be obtained by investing the sameamount in added pot lines using conventional self-baking anodes.

In a conventional self-baking anode system, a rectangular anode having ahorizontal cross section of, for ex-' ample, 63 inch width by 200 inchlength is suspended in a pot whose lining circuits the anode with aspacing of about 25 inches from each of the sides and ends of the anodeand the pot is run at an anode current density up to about 5 or 6 ampsper square inch.

The greater capacity of pot lines equipped with replaceable pre-bakedblock anodes stems from the greater anode current density at which suchanodes are able to run, typically about 8.5 amps per square inch, andthegreater total bottom area of an array of prebaked anodes asconventionally used, compared to the corresponding bottom area ofself-baking anodes, as conventionally used, in pots occupying the samefloor area.

The questions which may logically be asked at this point are: If onewished to avoid the expense of presses and furnaces, yet obtain for aplanned or existing selfbaking anode system pot line the capacity whichit would have if equipped with a pre-baked anode system, why not merelymake the self-baking anodes larger, so they occupy more of the pot crosssectional area, and why not run these larger anodes at an increasedcurrent density? The answers are founded upon some fundamentalconsiderations.

During the reduction of alumina to aluminum about one-half pound ofanode carbon is consumed, for each pound of aluminum produced. Gases,largely carbon dioxide, are formed at the anode as the carbon isconsumed. To the extent that these gases experience difficulty inexiting from the vicinity of the region between the anode and cathode,the capacity of the cell or its efficiency in producing aluminum isreduced.

Further, as alumina is reduced to aluminum more alumina must be added.To the extent that it is difficult to supply alumina to the pot and forthe alumina to work under the anode to utilize the whole bottom area ofthe anode, operating costs are increased and the efficiency of the potin producing aluminum is reduced.

Accordingly, merely increasing the size of conventional self-bakinganodes to achieve a total anode cross-sectional area comparable to thatof pre-bake cells and runningthe larger self-baking anodes at a currentdensity comparable to that conventional for prebaked anode systems hasbeen impractical because: (a) the gas venting path along the anodeworking surface would be increased, (b) the ore travel path countercurrent to the flow-of gas would be increased and (c) it would bedifficult to provide for feeding ore into the pot by conventional meansat the resulting narrow regions left between the larger anode and thepot lining. In addition, as regards the removable side channel casingconstruction commonly used with side-entry Soderberg cells, there wouldbe increased danger of excessive air burning of the anode due to thehigher anode current density.

SUMMARY OF THE INVENTION The present invention is directed toimprovements in the operation of alumina reduction cells having acontinuous electrode, particularly as regards techniques for feedingalumina into the bath of a reduction cell and procedures for collectingand removing anode reaction gases. It further relates to improved anodeconstructions in alumina reduction cells, as well as accessory apparatusfor feeding and gas collection purposes.

More particularly, the present invention concerns self-baking and otherforms of continuous anodes having a plurality of spacedly adjacent anodepassageways having their lower outlet ends located inside the outerperiphery of the anode working surface. Anode passageways in accordancewith the invention preferably are arranged and disposed so that no pointon the outer peripheral edge of the anode working surface is fartherremoved from the nearest passageway outlet than three-fourths the anodewidth. In this respect, the term anode width" refers to the narrowestouter cross-sectional dimension of an anode in the plane of its workingsurface. Some passageways may be used only for feeding and others forgas removal, or some or all may be used for both purposes; however, Iprefer to provide a plurality of passageways for feeding purposes inorder to achieve more uniform distribution of alumina in the bath,particularly in the region of the bath underlying the anode workingsurface.

As regards anode configurations of substantially rectangularcross-section in a horizontal plane, furthermore, especially thosehaving a longer dimension lengthwise of the cell than their width acrossthe cell, the anode arrangement includes passageways which are spacedlengthwise of the cell (i.e. along its longer dimension), and whichpreferably are disposed to satisfy the additional constraint that thedistance between their adjacent outlet ends is no greater thanthree-fourths the anode width across the cell.

In a preferred anode construction, the passageway outlets are sodistributed that every point on the anode working surface is no fartherremoved from the nearest outlet than three-fourths the anode width (asabovedefined).

It will be apparent that the foregoing arrangements are effective toreduce the paths of travel for feeding alumina beneath the anode and forventing anode reaction gases along the anode working surface. Inaddition, gas collection is readily accomplished by using hood meansassociated with said anode passageways or using supplemental hood meansalong the sides of the anode, or both.

By placing the aforementioned continuous anode or anodes in closeproximity to the sidewalls of the cell, furthermore, preferably at aclearance of a foot or less, it becomes possible to increase the totalanode area and consequently to improve the rate of production ofaluminum to a value more typical for cells using pre-baked anode blocks.In particular, an anode system of the type described herein may bearranged to provide an anode loading of the cell in which the aggregateanode cross-sectional area constitutes more than 50 percent of the floorarea occupied by the cell; and then a production rate exceeding 7lbs/day for each square foot of floor area can be achieved.

The method aspects of the present invention include feeding alumina intothe bath through the anode passageways, and maintaining a blanket ofalumina on the crust adjacent exposed surfaces of the anode to protectthe anode carbon against air burning. It can be seen, of course, thatfeeding at spaced locations within the outer periphery of the anode canbe utilized to achieve better distribution of alumina beneath the anodethan would be the case using feed sites located along the sides of thecell. Feeding through the anode also makes it possible to place theanode closer to the sidewalls of the cell, so as to achieve a higherratio of anode to pot shell area and consequent increase inproductivity.

The feeding of alumina into the bath as operation of the cell proceedsis preferably carried out with the least possible disruption of theusual crust of solidified electrolyte which forms over the bath andaround the anode. For one reason, as previously noted, this crust may beused to support a blanket of alumina adjoining the anode for protectionagainst air burning. Since crust may also form at the lower outlet endsof the anode passageways, a system of plungers or other means areprovided for maintaining holes through the crust adjacent the passagewayoutlets and the alumina requirements of the cell are preferablyintroduced through such passageways and holes rather than by breadingmassive portions of the crust outwardly of the anode. Anode reactiongases or a portion thereof may also be released from the cell throughthese holes and the adjoining anode passageways.

Most of the anode gases (from 50 to percent) are released at the sidesof the anodes through enclosures which occupy a small portion of theperipheral space between the anode and the pot sidewall. Plunger meansare provided to maintain communication between the gas collectingenclosures and the space beneath the pot crust, without disturbing thegas seal between the enclosures formed by frozen electrolyte and theblanket of ore resting on the frozen electrolyte. In addition, thepractice of the present invention preferably includes feeding aluminainto the bath at frequent intervals and at spaced locations; but mayinvolve feeding at one location and then another, rather than feedingsimultaneously at different locations. A further benefit of this feedingarrangement is that it allows for maintenance ofa blanket of alumina onthe crust between the anode and adjacent sidewall of the cell to protectthe anode against air burning.

Side-entry pins, top entry pins or a combination of the two may be usedfor establishing electrical connection between the bus flexes and theanode.

The invention may be practiced with anodes formed of Soderberg paste,continuous cemented prebaked carbon, or continuous cemented semi-bakedcarbon.

VERTICAL PIN SODERBERG In the case of a vertical pin Soderberg potemploying a fixed steel casing around the anode, the present inventionmay include provision for supplemental collection of anode gases bymeans of a hood or other similar enclosure attached to the anode casing,which occupies a substantially smaller portion of the anode peripherythan the usual complete skirts. With such a hood arrangement, plungermeans are provided for maintaining holes through the crust within thegas collection enclosures. These plungers are conveniently activated bya common beam traversing the length of the anode and moved by pneumaticor mechanical means at the ends of the pot. Alumina may also beintroduced through an inlet into the enclosure and onto the crusttherein.

We find it advantageous both from the standpoint of efficient potoperation and from the standpoint of minimizing the hardware needed tobreak the crust, to actuate the plungers frequently; for example, onceevery I to 5 minutes. In this way the crust never forms inside the gascollection enclosures to an extent that it offers high resistance tobreakage and lighter mechani-' described generally above, it thenbecomes unnecessary to routinely break the crust that forms between thesidewalls of the pot and the anode. Alumina can be placed upon thiscrust in deep blanket to protect the anode carbon from attack by air andincrease the effective area of carbon presented for electrolysis,thereby improving the efficiency of the cell and its ability to carryhigh amperage. In contrast, former methods of collecting gas ordinarilyrequired breaking the crust outside the periphery of the anode or theanode gas skirt for purposes of feeding alumina. This action destroyedthe effectiveness of the gas skirts and permitted air to attack theanode, reducing its area and causing carbon particles to fall off intothe electrolyte where they caused a reduction in the efficiency of thecell.

SIDE PIN SODERBERG For use with a side pin Soderberg anode, for example,a supplemental gas collection enclosure may be mounted-on thepot shell,so that its bottom edge is spaced from the liquid electrolyte, in theregion between the anode and the sidewall, by a distance which issufficient distance to avoid consumption of the materials from which theenclosure is made, but close enough to the electrolyte that a crust offrozen electrolyte naturally forms between the enclosure and adjacentsidewalls of thepot and also between the enclosure and adjacent side ofthe anode. Within the enclosure, holes are provided through the crustfor removing the anode gases evolved from the cell.

CONTINUOUS CEMENTED BLOCKANODES block anodes that no portion of them hasto be removed routinely. This means that the pot crust does not have tobe broken for routine anode replacement, thus eliminating a troublesomesource of fume from the pot. Continuous prebaked electrodes have thefurther advantage over pre-baked block anodes that no portion of thecarbon is returned to the carbon plant for recycling. This means that,with continuous prebaked anodes, electrolyte materials are not returnedto the carbon plant where they adversely affect refractory life inbaking furnaces and add to the difficulty of scrubbing baking furnacegases efficiently.

The continuous semi-baked anodes have the advantages of the continuouspre-baked and have the further advantage that final baking temperatureis in the range 400 to 600 C. as opposed to l,I00'C. for

fully pre-baked blocks. This means that baking furnaces can bemuchsmaller and less expensive to build and maintain. It also means thattunnel kilns can be used advantageously.

Continuous pre-baked anodes in which the blocks have been fullypre-baked (I,l00 C.) before addition to the pot use short anode contactpins, cemented into holes which have been drilled into the baked anode.The distance electric current must travel from these pins to the workingsurface of the anode is greater than for side pin Soderberg pots withthe same anode width. This results in higher voltage losses for thecontinuous pre-baked anode. This disadvantage gets more severe as onewidens the anode to obtain the benefits achievable with center feedingas taught in this invention.

Semi-baked slabs have the advantage over fully baked slabs that longpins, typically as long as are used for Soderberg anodes, can be formedinto the green blocks and left in the blocks during the furnacetreatment to drive off pitch and establish a rigid block structure.

Typical Forms of Anode, Feed, and Gas Vent Locations Illustrating theInvention with Single In cells having continuous pre-baked or semibakedanodes, the gas collection enclosure may be supported as in the casewith the gas collection and feeding systems previously mentioned for theside pin Soderberg pot. Plunger means may be associated with thehood-like enclosure to maintain communication between the space belowthe crust and the gas collecting hood, so that anode gases can becollected from this hood. Here again it is not necessary to disturb thecrust and ore blanket cover between and around the carbon anodes duringat least a major portion of the operating cycle,thereby reducing theoccasions for the attack of air on the anode carbon and avoiding thenecessity of disturbing the pot crust outside the enclosure, either forpurposes of feeding'the pot or removing gas.

In the various anode systems described it will be advantageousto providemeans of access to the area inside the supplemental gas collectionhoods. In the case of a hood mounted from a permanent anode casing, asin a vertical pin Soderberg cell, it is convenient to provide slidingaccess doors. In the case of a hood for the side pin Soderberg pot,either hinged doors may be provided or else the hood itself may behinged to the deckplate of the pot.

For purposes of further comparison it may be noted that typicalconventional side pin self-baking anode systems employing single anodescarry up to about amps for each square inch of anode bottom surface, andabout 2.5 amps for each square inch of shell bottom area (which providesan estimate of aluminum reduction plant capacity per unit area of plantfloor space).

Typical pots using pre-baked block anodes carry up to about 8.5 amps foreach square inch of anode bottom area, and about 4 amps for each squareinch of pot shell bottom area.

In contrast, pots equipped with a continuous anode system and operatedin accordance with the present invention may carry about 7.5 amps foreach square inch of anode bottom area, and may be arranged to occupy asubstantially greater part of the available shell area than conventionalSoderberg anodes. Thus, although alumina reduction cells havingpassageways through a self-baking anode have been known for some time,the art has not previously recognized how to achieve full benefits inthe construction and operation of such cells particularly as regards theinclusion of anode passageways spaced lengthwise of the cell anddistributing the passageways outlets substantially uniformly over theanode working surface.

For further comparison, typical side pin Soderberg pots with anodescarrying 80,000 amps, for example, are hooded to capture hydrocarbon andfluoride gases for scrubbing at a rate of about 5,000 cfm per pot. Incontrast, continuous anodes, carrying the same current whetherSoderberg, semibaked, or pre-baked employing this invention capture thefluoride-bearing gases and most of the gases resulting fromdecomposition of pitch binder with a movement of about 500 cfm per pot.

.Thus, scrubbing can be much more efficient with a given investment,because more concentrated gases are scrubbed and less air is handled toaccomplish the removal of potential air pollutants.

DESCRIPTION OF THE DRAWINGS The invention is further discussed hereinwith reference to the drawings wherein the presently preferredembodiments are shown. The specifics illustrated in the drawings areintended to exemplify, rather than limit, aspects of the invention asdefined in' the claims.

FIG. 1 is a perspective view (partly in section) showing a Soderbergcell arranged for operation in accordance with the invention;

FIG. 2 is a similar perspective view of a reduction cell having acontinuous cemented slab anode and associated apparatus of the presentinvention; and

FIGS. 3 and 4 are detailed sectional views of portions of the apparatusshown in FIGS. 1 and 2, respectively.

FIG. 1 illustrates an embodiment of the invention in which the cellcomprises a vertical pin Soderberg electrode arranged for center ventingand feeding of alumina, as well as for supplementary peripheral ventingof anode gases. The pot shell 10, cathode lining 12 and cathodecollector bars 13 are conventional. The anode is formed in a permanentcasing 14, supported by rods 15 from suitable frame members (not shown),to allow freedom of movement upwardly when the anode is raised, whileproviding support for the casing in its lowermost position.

Anode pins 19 are clamped for electrical and mechanical connection byclamps 20 to movable bus 21. Supplementary clamps 22 also connect theanode pins to the bus through a flexible connector 23. The combinationof clamps 20 and 22 permits periodic upward movement of the bus 21 withrespect to the pins 19 without interruption of current flow to the pins.

Each of the alumina feeders 16 (later detailed in connection with FIG.3) has tubular casing means extending approximately to the level of thebake zone of the carbon to provide a passageway through the anode, andfurther includes plunger means adapted to penetrate any crust that mayform within the anode passageway at its lower outlet end adjacent thebath. The feeder means are arranged to receive alumina from an ore bin17, as hereinafter described more fully.

The feeder assemblies, which also include means for removing reactiongases liberated through the anode passageways, are spaced along thecenterline of the anode at distances such that no point on the anodeworking surface is farther removed from a point of feeding thanthree-fourths the anode width.

Supplementary gas collection enclosures 30 are mounted to the permanentcasing 14 in such a way that they are supported with their lower edgesabout 4 to 6 inches above the level of liquid electrolyte in the pot andlow enough to be enclosed within the ore blanket 31 which covers theelectrolyte normally freezing between the anode casing and the adjacentsidewall of the cell. Plunger means 32 are provided to maintain anopening in the crust within the gas enclosure 30.

Gas collection tubes 33 convey evolved gases from the enclosures 30 to ascrubbing system.

Secondary shields 34 are provided to capture any hydrocarbon fume thatis evolved during operations to replace pins 19. This system providesthe opportunity to capture and scrub concentrated fluoride gasesseparately from the dilute hydrocarbon-bearing gases captured bysecondary hoods 34. The feeding and fume collection arrangementdescribed makes it possible to feed alumina to the cell and collectgases from beneath the anode without disrupting the ore cover 31. Wehave found that it is sufficient to have relatively small enclosures 30adequately spaced around the cell instead of complete gas collectionskirts as previously practiced in vertical pin Soderberg pots. Servicingof the anode with respect to anode pin replacement and paste addition isas normally practiced in vertical pin Soderberg operations.

FIG. 3 illustrates in detail the feeder/gas collection mechanism 16 ofFIG. 1. A steel sleeve 41 is supported in the ore bin 17 by the bracemembers 44. That sleeve has lateral openings 45 to admit alumina fromthe bin. Within sleeve 41 is a rotatable sleeve 46 resting upon a fixedsupport ring 47 which is attached to sleeve 41. After the installationof sleeve 46, a collar 48 is installed to which is affixed a lever arm49. Sleeve 46 supports a transverse plate 50 incorporating outlet holes51 for passage of alumina and a central opening 52 for the breaker rod53. The breaker rod is attached to piston 54 which operates in cylinder55, and an inner sleeve 56 is attached to the bottom of cylinder 55.Both cylinder 55 and sleeve 56 are mounted so that they cannot rotate.

Sleeve 56-supports a lower plate 57 having holes 58 for passage ofalumina. Sleeve 46 has lateral openings 59 for passage of alumina. Inthis arrangement the entire assembly comprising the cylinder 55, sleeve56 and sleeve 46 can be placed within sleeve 41 after which the collar48 is affixed.

Tube 59 admits air for the downward thrust of the breaker rod 53. Tube60 provides air for the return stroke.

The sleeve 46 can be rotated by means of an air cylinder (not shown)acting against the lever 49. The inlet holes 45 and outlet holes 58 areso oriented that when holes 45 are open, holes 58 are closed. Thisarrangement provides a metering chamber in the annular space betweensleeves 46 and 56 to hold a charge of alumina until it is dumped. Inorder to dump ore, lever 49 is activated to close port 45 and open port58. On the return stroke port 58 is closed, port 45 is opened and a newcharge of ore is measured out. The action which dumps ore is a rotatingof sleeve 46 within sleeve 41. I

The collar 48 shaped so that leakage of alumina can be stopped due toits angle of repose, without requiring a tight fit. Operation ofthebreaker rod 53 is conveniently controlled through a timer mechanism(not shown) separate. from the mechanism which.

operates the ore dump control lever 49. While it is possible to makethese two actions coincident, it is contemplated that under mostconditions they will be performed independently. For example the breakerrod may be activated once every five to ten minutes and the ore dumpmechanism may be activated once everyhalf hour.

An anode sleeve 61 is provided which fits loosely into the chamber 63.Such an exhaust port would be adjustedto discharge 'airfrom the upperchamber 'of the cylinder whenthe piston 54,reaches the port position.Alternatively, the plunger shaft 53 may be threadedthrough the piston 54and extend outwardly at the top of the cylinder for adjustment purposes.

FIG, 2 illustrates an alternate embodiment of the in-v vention in whichthe cell comprises a continuous, cemented, semi-baked anode with sidepin electrical contacts, center venting andfeeding of alumina, andperipheral venting of concentrated anode gases. Pot shell 10, lining 12,and cathode collectors 13 are conventional. The anode 74 is comprised ofextruded or rammedcarbon blocks 75 which have been baked to atemperature sufficient to develop their structural strength and driveoff the pitch volatiles in the furnace. This temperature is betweenabout 400and about 600 C, depending upon the mix used in forming thecarbon block. The baking can be donein a tunnel kiln under conditionssuch that a goodportion of the evolved pitch is condensed andavailablefor re-use in carbon forming operations. The carbon blocks 75are cemented together by carbonaceous paste at the interface 76, and, asthe anode is consumed and the blocks are lowered, their baking iscompleted in place, but without evolution of pitch fume except for thatpresent in the cementing paste, and any additional paste used to sealaround the ore feeding breakers as hereinafter discussed.

Anode contact pins 77 are long, typically of the length used for sidepin Soderbergs. They are inserted in the blocks before the blocks areplaced in the baking furnace. Since the bocks are not baked above 600 C,the steel pins are not affected by the temperatures in the bakingfurnace. The pins are later removed in accordance with conventional sidepin practice. Pins 77 are connected through conductors 78: to the anodebus 21, using clamps 80. Pin changing and connector raising proceduresare as typically performed in side pin Soderberg operations.

The ore feeders 81 are sealed to the anode blocks by means ofcarbonaceous paste or mixtures of alumina and cryolite. Each feeder(later described in detail with respect to FIG. 4) has an activatingmechanism for its plunger 83, and includes ore inlet tube 84. To provideaccess for additional blocks 75 as the anode is consumed, the spaceabove the blocks is left open and the ore bin normally used is replacedby conveyor means 85. When blocks are to be replaced, ore feeders 81 aredisengaged and raised to a new higher position to accommodate the addedheight of the new blocks, and are re-sealed to the anode in thisposition to prevent flow of gas between the casing of the feeder and theadjacent carbon.

The ore feeders are so located that no point on the workingsurface ofthe anode is farther removed from a point of venting and ore feedingthan three-fourths the anode width. Gas collection tubes 86 are providedto remove that portion of anode gases evolved through the anode at thefeed sites. Supplementary gas collection enclosures 87 are suspendedabove the electrolyte at a distance of about 4 to 6 inches by means ofcantilever arm 88 supported by pivot and block 89. Breaker bars 91operate inwardly of the enclosures 87 to maintain an access hole throughthe crust, admitting gases from beneath the, crust to the enclosures,from which theyare withdrawn byway of tube 92. The breaker 91 isheight-adjustable, so that its lowest point of movement is aboveimmersion in the electrolyte. By this action, cryolite does not freezeand adhere to the breaker bar, and the hole it cuts through the crust isapproximately equal in shape to the cross-section of the bar. Sufficientvent locations are provided to approximately match the alumina feed,locations. Concentrated fluoride gases are collected through tubes 86and 92, and conveyed to scrubbers. The volume of gas which must becollected under this system in order to properly vent the pot is on theorder of one-tenth the volume of gas collected in a system with generalhooding around the anode as would be practiced in conventional side pinSoderberg pots. The concentration of fluorides is correspondinglygreater and therefore the efficiency of the scrubbers is greater for agiven amount of gas movement horsepower installed.

The gas collection enclosure 87 is sealed in the alu- -mina blanket 31which rests upon the crust that normally forms over the electrolyte. Acomplete ore blanket is maintained under all routine conditions suchthat alumina is banked up against the electrode to prevent air burning,and this blanket is not disturbed for the purposes of gas venting oralumina feeding.

It can be seen that the embodiment of the invention as illustrated inFIG. 2 can be practiced also for fully pre-baked carbon slabs, underwhich conditions it has been found more practical to drill holes in thecarbon for the insertion of short anode pins as opposed to the insertionbefore baking of long anode pins as in the case where semibaked carbonsare used.

It can further be seen that the feeder and gas collection elementsillustrated in FIG. 2 can be practiced in conventional side pinSoderberg pots.

Turning next to FIG. 4, the feeder/venting mechanism 81 will bedescribed in greater detail.

FIG. 4 illustrates an alternative breaker and ore feeding arrangement asprovided in the reduction cell of FIG. 2. In FIG. 4, cylinder 101 isfixed to a beam 102 supported by the cells superstructure. Anode sleeve104 slides over cylinder 101 and is set into a recess 105 of theprebaked block 75 after the block is installed. Carbon paste or alumina107 is used to make a gas seal between the sleeve 104 and the anodeblock. Piston 108 operates plunger 83which is preferably heightadjustable (as previously discussed) so that the plunger does not touchthe bath in its lowest position. Alumina is transported through duct 85from which it discharges to metering device 111, and then through duct84 to the anode. The shaft of the metering device is turned by a motorand associated clock mechanism (not shown), and piston 108 is operatedby air from a valve control which is responsive to a separate actuatingmechanism. The two control mechanisms may be set to give coincidentoperation of ore metering device 111 and plunger 83; however, in mostcases, they would be operated independently as previously discussed inconnection with FIG. 3. Tube 86 is provided to receive anode gasespassed outwardly of the cell through the anode passageway.

With respect to the anode blocks 75 semicircular holes are formed in theside of each anode segment to provide for putting the segments in placearound the feeder device.

As the electrode is consumed sleeve 104 slides down over sleeve101. Whenit comes time to place a new block over anode block 75, sleeve 104 isdisengaged from the sealing medium 107 and raised. After the new blockis in place, the sleeve is set back down in the recess provided andrescaled.

While the presently preferred practices of the invention have beenillustrated, described and discussed, it will be apparent that theinvention may be otherwise variously embodied and practiced within thescope of the following claims.

What is claimed is:

1. In an alumina reduction cell having a bath of molten electrolytecontaining dissolved alumina, a crust of solidified electrolyteoverlying the bath and means for passing electric current through thebath, the improvement comprising:

a. a continuous anode having a plurality of tubular passagewaysextending downwardly through the anode at spaced locations within itstransverse cross-section;

b. means for breaking an opening through any crust formed in saidpassageways at their lower outlet ends adjacent the anode workingsurface; and c. means for introducing alumina into the bath through saidpassageways.

2. Apparatus according to claim 1 including means for collecting gasesevolved from the cell through said anode passageways.

. 3. Apparatus according to claim 2 in which said passageways are sodisposed that no point on the outer periphery of the anode workingsurface is further removed from the nearest passageway outlet thanthree-fourths the anode width.

4. Apparatus according to claim 1 including an anode having passagewayswhich are spaced lengthwise of the cell.

5. Apparatus according to claim 4 having anode passageways spacedlengthwise of the cell along its centerline.

6. Apparatus according to claim 4 in which said anode is longerlengthwise of the cell than its width across the cell.

7. Apparatus according to claim 1 in which said passageways are sodisposed that every point on the anode working surface is no furtherremoved from the nearest passageway outlet than three-fourths the anodewidth.

8. Apparatus according to claim 1 in which the aggregate anodecross-sectional area constitutes more than 50 percent of the floor areaoccupied by the cell.

9. Apparatus according to claim 1 in which said anode comprisescarbonaceous blocks which are baked prior to installation to eliminatesubstantially all hydrocarbon fumes and to develop sufficient structuralstrength to support themselves in service in the anode.

10. Apparatus according to claim 9 in which said carbonaceous blocks arebaked at a temperature between 400 and 600 C.

l 1. Apparatus according to claim 10 in which electrical contact pinsare inserted in the anode blocks before baking.

12. In an alumina reduction cell having a bath of molten electrolytecontaining dissolved alumina, a selfbaking carbon anode including abaked lower portion adjacent the bath and a softer upper portion, and acrust of solidified electrolyte overlying the bath, the improvementcomprising:

a. tubular casing means in at least said upper portion of the anode toprovide interior passageways extending downwardly through the anode; and

b. plunger means operable inwardly of said passageways to preventobstructions therein due to crust formations at the lower outlet endsthereof adjacent the anode working surface.

13. Apparatus according to claim 12 including feeder means forintroducing alumina into the anode passageways, said plunger means beingarranged for movement between a lower position to produce anaccess'opening through any crust formed at said lower outlet end of eachanode passageway and an upper position allowing alumina to flow into thebath through said access opening.

14. Apparatus according to claim 12 in which said passageways andassociated plunger means are so disposed that no point on the outerperiphery of the anode working surface is farther removed from thenearest access opening than three-fourths the anode width.

15. Apparatus according to claim 14 including means for collecting gasesevolved from the cell through said passageways.

16. In the operation of an alumina reduction cell having an anode, abath of molten electrolyte containing dissolved alumina and a crust ofsolidified electrolyte overlying the bath, the method which comprises:

providing a continuous anode having a plurality of interior passagewaysextending downwardly through the anode, said passageways having theirlower outlet ends disposed adjacent the anode working surface;

feeding alumina passageways;

maintaining a blanket of alumina and supporting crust around the anodeto provide a substantially gas-tight cover over the bath as said feedingproceeds;

breaking an opening through any crust formed within said passageways atthe lower outletends thereof; and

collecting gases evolved from the cell through said passageways, wherebysaid feeding and gascollecting operations are carried out withoutdisrupting the main body of said crust outwardly of the anode.

17. The method of claim 16 which includes providing access openingsthrough the crust at spaced locations outwardly of the anode, saidopenings being spaced apart sufficiently to preserve the integrity ofthe crust into the bath through said between openings; and collectinggases evolved from the cell through said openings in the crust,

18. The method of claim 16 which includes providing a continuous anodein which said passageways are so disposed that no point on the outerperiphery of the anode working surface is farther removed from thenearest passageway outlet than three-fourths the anode width.

19. The method of claim 16 which includes providing a continuous anodehaving passageways which are spaced lengthwise of the cell.

20. The method of claim 16 which includes providing a continuous anodewhich is longer lengthwise of the cell than its width across the cell.

21. The method of claim 16 which includes providing a continuous anodein which said passageways are so disposed that every point on the anodeworking surface is no farther removed from the nearest passageway outletthan three-fourths the anode width.

22. The method of claim 16 in which said feeding comprises passingalumina into the bath from the lower outlet end of one or more of saidpassageways but includes repeated feeding at spaced locations.

23. The method of claim 22 in which said feeding occurs at intervals andin amounts sufficient substantially to compensate for depletion ofalumina from the bath.

24. In an alumina reduction cell having a bath of molten electrolytecontaining dissolved alumina, a crust of solidified electrolyteoverlying the bath and an anode for passing electric current through thebath, the improvement comprising: l

a. a continuous anode having a plurality of interior passagewaysextending downwardly through the anode, said passageways being sodisposed that no point on the outer periphery of the anode workingsurface is farther removed from the nearest of said passageways at itslower outlet end than threefourths the anode width;

b. means for collecting reaction gases of the cell through openings inthe crust outwardly of the anode at spaced locations which are separatedsufficiently to preserve integrity of the crust as a substantiallygas-tight cover over the bath;

c, means for feeding alumina into the bath through said interiorpassageways of the anode, including means for breaking an openingthrough any crust formed within said passageways at their lower outletends adjacent the bath; and

(1. means for collecting reaction gases evolved from the cell throughsaid anode passageways.

1. In an alumina reduction cell having a bath of molten electrolytecontaining dissolved alumina, a crust of solidified electrolyteoverlying the bath and means for passing electric current through thebath, the improvement comprising: a. a continuous anode having aplurality of tubular passageways extending downwardly through the anodeat spaced locations within its transverse cross-section; b. means forbreaking an opening through any crust formed in said passageways attheir lower outlet ends adjacent the anode working surface; and c. meansfor introducing alumina into the bath through said passageways. 2.Apparatus according to claim 1 including means for collecting gasesevolved from the cell through said anode passageways.
 3. Apparatusaccording to claim 2 in which said passageways are so disposed that nopoint on the outer periphery of the anode working surface is furtherremoved from the nearest passageway outlet than three-fourths the anodewidth.
 4. Apparatus according to claim 1 including an anode havingpassageways which are spaced lengthwise of the cell.
 5. Apparatusaccording to claim 4 having anode passageways spaced lengthwise of thecell along its centerline.
 6. Apparatus according to claim 4 in whichsaid anode is longer lengthwise of the cell than its width across thecell.
 7. Apparatus according to claim 1 in which said passageways are sodisposed that every point on the anode working surface is no furtherremoved from the nearest passageway outlet than three-fourths the anodewidth.
 8. Apparatus according to claim 1 in which the aggregate anodecross-sectional area constitutes more than 50 percent of the floor areaoccupied by the cell.
 9. Apparatus according to claim 1 in which saidanode comprises carbonaceous blocks which are baked prior toinstallation to eliminate substantially all hydrocarbon fumes and todevelop sufficient structural strength to support themselves in servicein the anode.
 10. Apparatus according to claim 9 in which saidcarbonaceous blocks are baked at a temperature between 400* and 600* C.11. Apparatus according to claim 10 in which electrical contact pins areinserted in the anode blocks before baking.
 12. In an alumina reductioncell having a bath of molten electrolyte containing dissolved alumina, aself-baking carbon anode including a baked lower portion adjacent thebath and a softer upper portion, and a crust of solidified electrolyteoverlying the bath, the improvement comprising: a. tubular casing meansin at least said upper portion of the anode to provide interiorpassageways extending downwardly through the anode; and b. plunger meansoperable inwardly of said passageways to prevent obstructions thereindue to crust formations at the lower outlet ends thereof adjacent theanode working surface.
 13. Apparatus according to claim 12 includingfeeder means for introducing alumina into the anode passageways, saidplunger means being arranged for movement between a lower position toproduce an access opening through any crust Formed at said lower outletend of each anode passageway and an upper position allowing alumina toflow into the bath through said access opening.
 14. Apparatus accordingto claim 12 in which said passageways and associated plunger means areso disposed that no point on the outer periphery of the anode workingsurface is farther removed from the nearest access opening thanthree-fourths the anode width.
 15. Apparatus according to claim 14including means for collecting gases evolved from the cell through saidpassageways.
 16. In the operation of an alumina reduction cell having ananode, a bath of molten electrolyte containing dissolved alumina and acrust of solidified electrolyte overlying the bath, the method whichcomprises: providing a continuous anode having a plurality of interiorpassageways extending downwardly through the anode, said passagewayshaving their lower outlet ends disposed adjacent the anode workingsurface; feeding alumina into the bath through said passageways;maintaining a blanket of alumina and supporting crust around the anodeto provide a substantially gas-tight cover over the bath as said feedingproceeds; breaking an opening through any crust formed within saidpassageways at the lower outlet ends thereof; and collecting gasesevolved from the cell through said passageways, whereby said feeding andgas collecting operations are carried out without disrupting the mainbody of said crust outwardly of the anode.
 17. The method of claim 16which includes providing access openings through the crust at spacedlocations outwardly of the anode, said openings being spaced apartsufficiently to preserve the integrity of the crust between openings;and collecting gases evolved from the cell through said openings in thecrust.
 18. The method of claim 16 which includes providing a continuousanode in which said passageways are so disposed that no point on theouter periphery of the anode working surface is farther removed from thenearest passageway outlet than three-fourths the anode width.
 19. Themethod of claim 16 which includes providing a continuous anode havingpassageways which are spaced lengthwise of the cell.
 20. The method ofclaim 16 which includes providing a continuous anode which is longerlengthwise of the cell than its width across the cell.
 21. The method ofclaim 16 which includes providing a continuous anode in which saidpassageways are so disposed that every point on the anode workingsurface is no farther removed from the nearest passageway outlet thanthree-fourths the anode width.
 22. The method of claim 16 in which saidfeeding comprises passing alumina into the bath from the lower outletend of one or more of said passageways but includes repeated feeding atspaced locations.
 23. The method of claim 22 in which said feedingoccurs at intervals and in amounts sufficient substantially tocompensate for depletion of alumina from the bath.